The pressure in the pressure vessels generally will be measure by a pressure gauge in the prior art. The operational principle of the common pressure gauge is as follows: a displacement of the free end of the spring tube will be caused when the air in the pressure tube enters the C-shaped spring tube (the spring tube will expand outward when the measured pressure is larger than the atmospheric pressure, and the spring tube will contract inward when the measured pressure is smaller than the atmospheric pressure), then the air will pass mechanisms comprising the driving and zooming mechanism, the fan-shaped gear and the mini gear, etc. to drive the needle to deflect, and thus indicating the amount of the pressure. Such a pressure gauge has a complex structure and a high cost.